Birds and Declivity Currents

Wednesday, November 18, 2009

Nature Notes
by Bob Thomas

Animals have all sorts of modes of movements, and a knack for finding extraordinary efficiencies in their chosen form of locomotion is physiologically rewarding.

For birds, equipped with their aerofoil wings that keep them afloat in the air, their ability to find and take advantage of moving air is a plus.

An excellent example may be seen when crossing Lake Pontchartrain on windy days.

Wind blows unobstructed across the lake until it is deflected upward by structures like a span of bridges. Several species of birds, especially brown pelicans, herring and ringbill gulls, and the occasional tern, use these deflected winds, called declivity currents, to their advantage by using them to lift themselves. Riding these deflected winds is called declivity, or slope, soaring and the birds appear to fix their wings and glide just above the rail of the bridge. They face slightly toward the wind, thus ensuring lift as both the deflected winds hit their wings from below and the winds crossing above the bridge flow over the aerofoil shape of their wings.

Recently I saw several brown pelicans gliding northward on the Causeway. They moved effortlessly and rather quickly. There was not a terribly strong wind, but they were gliding at about 40 mph. This is fast for such a big bird.

As cars approached them, they veered off the rail to the east, and steered back over the rail after cars passed.

Birds may also use this form of soaring along the sides of boats that are under way offshore.

Declivity currents abound along the ridges of the Appalachians. Thousands and thousands of raptors (birds of prey such as hawks, falcons, and eagles) use Atlantic winds deflected upward by the Appalachian Mountains as the birds migrate south in the late summer and fall. Hawk Mountain is a not-for-profit organization that offers people interested in raptors educational and volunteer observation opportunities that are annually made possible by a combination of declivity currents and the innate urge for the northern dwelling raptors to migrate south each winter.

Slope soaring birds along the Causeway and Twin Spans are an excellent example of how animals adapt to living in a complex world of dynamic elements of their ecosystem. Most people just speed by and take no notice of the bird’s reliance on the physics of wind movement.


A brown pelican gliding along declivity currents                            A brown pelican riding declivity currents
over the east rail of the Causeway over Lake                                along the I-10 Twin Spans over
Pontchartrain. Note the bird to the right that has                          Lake Pontchartrain.
left the rail as the car approaches.                                               Photo by Shannon Fortenberry.
Photo by Bob Thomas.